Epitaxy is a process by which one or more crystalline overlayers are deposited on a crystalline substrate (e.g. sapphire). Various processes for fabricating sapphire substrates (wafers) and related devices have been developed, as described in U.S. Pat. Nos. 9,449,818, 9,455,374, 7,558,371, 8,226, 767 and U.S. Patent Publication No. 20140264459. Epitaxy processes involving semiconductor materials such as cubic or zinc blende on a sapphire substrate (wafer) typically require the sapphire substrate/wafer to be at a high temperature in the range of about 600° C. to about 1000° C. For example, the growth of a GaN epitaxial layer on a sapphire substrate for LED fabrication may require a sapphire substrate temperature of about 850° C. Forming an SiGe epitaxial layer on a sapphire substrate may require a sapphire substrate temperature of about 890° C. Known methods to heat sapphire substrates may suffer from problems such as low yield and/or high production cost. For example, if the heat absorption rate of a sapphire substrate is low, heating the substrate to the desired temperature may be slow, leading to increased expense.
Because heating of sapphire substrates/wafers generally occurs in a vacuum environment, the sapphire substrates or wafers cannot be heated utilizing conductive or convective heat transfer. Thus, radiative heat sources/transfer must be utilized. However, because the sapphire substrates/wafers may be optically transparent, the sapphire substrates/wafers do not readily absorb radiative energy.
There are at least two known processes for heating sapphire substrates for epitaxial processes. First, the sapphire substrates may be positioned in a heated nitrogen or argon charged vacuum chamber for several hours to raise the temperature of the sapphire substrates. A plurality of the sapphire substrates may be positioned in the chamber to provide a batch process. Another known method of heating sapphire substrates involves coating a sapphire substrate/wafer with a metallic or carbon film utilizing a sputtering process. The coating may be deposited on a back surface of a sapphire substrate/wafer. The coating may be light-absorbing, and the coating can therefore absorb thermal radiation from a heat source to heat the sapphire substrate/wafer.
Because sapphire is optically thin and transparent, it may be difficult to raise the temperature of a sapphire substrate/wafer in a vacuum environment to a desired (uniform) level over the entire area of the sapphire substrate. Although thermal energy can be transferred from a heating element to a sapphire substrate/wafer by conduction through direct contact, direct conduction may cause uneven heating. Thus, conductive heating is prone to hot spots, thermal stresses, and subsequent breaking of the sapphire substrates/wafers. Convection is impossible in vacuum. Conduction and convection are therefore not viable methods for heating sapphire substrates/wafers in a vacuum for epitaxial processes.